NP uptake by 3D LUHMES and BrainSpheres. Representative images acquired by confocal microscope after 72 h exposure to 20 μg/mL PLA-NP (green) of a BrainSpheres stained with MAP2, GFAP, OLIG-1 (red) and Hoechst33342 (blue nuclei), with the Scale bars 100 μm (left panels) and 25 μm (right panels); and b 3D RFP-LUHMES (red), PLA-NP (green), scale bar 20 μm. c Flow cytometry of RFP-LUHMES, treated with 0, 0.2, 2 and 20 μg/mL PLA-NP. Red and green fluorescence was quantified in FL2 and FL1 channels, respectively. The bars represent % of RFP/PLA double positive cells in each sample. d Intracellular levels of Au in 3D LUHMES and BrainSpheres after 24 and 72 h exposure to 6 μg/mL Au-SC and 20 μg/mL Au-PEG quantified by ICP-MS. Results are expressed as mean (±SEM). Each experimental group corresponds to the analysis of three independent experiments with three replicates. Student’s t-test was used to compare 24 and 72 h treatment groups (*p < 0.05, ***p < 0.001).